1. Field of the Invention
The present invention relates to electronic devices, particularly methods for integrating fluid cooling systems on electronic devices.
2. Description of the Related Art
Electronic components and assemblies can generate large amounts of waste heat during operation. If this heat cannot be efficiently removed the temperatures of the electronic components and assemblies can rise to levels that prevent their normal function and can also cause failure. This issue is particularly severe for electronic applications that dissipate high levels of waste heat or operate in high temperature ambient conditions.
Current solutions may involve removing heat by using one of four heat removal mechanisms or a combination of the four mechanisms. These mechanisms include conduction, convection, radiation, and evaporation.
Conduction is the property of materials to conduct heat energy from hot to colder locations. This is a low cost and reliable method of heat removal. However, the amount of heat that can be removed is limited by the intrinsic thermal conductivity of the materials used and the amount of area in the material available to remove the heat. These limitations become even more important when the size of the component and assembly becomes smaller and as the density of heat generated at both the component and assembly level increases.
Radiation is the property of materials to radiate heat energy from hot to colder environments. The efficiency of radiation for heat removal is dependent on a number of variables. The most important of these is the difference in temperature between the hot surface and cold environment. In many electronic applications, this temperature difference is too low to allow for efficient transfer of heat energy away from the heat generating components.
Evaporation of fluids to vapor (gaseous phase) can be a very efficient method of heat removal. As a fluid evaporates, heat energy is required to transform the fluid into a vapor. The required heat energy is known as the latent heat of vaporization. This property has been used in devices such as heat pipes, thermosyphons, and refrigeration systems. The biggest issue with all of these systems is the need for the containment of the vaporized fluid over the life of the product. Heat pipes and thermosyphons may also require proper orientation with respect to gravity for proper operation. This can result in high packaging cost of these systems. Because most of these cooling concepts use relatively small amounts of fluid, in high heat applications “dry out” may happen when all of the fluid available is vaporized. When this happens, the system is no longer capable of removing heat from the electronic component or assembly, potentially causing system failure.
Convection is the property of heat transfer through moving fluids (liquids and gases) to remove heat from surfaces. The heat is absorbed in the fluid by the specific heat property of the fluid and is carried away from the hot surface as the fluid moves away. In some liquid cooling systems heat can be absorbed by the formation of small vapor bubbles in the fluid caused by partial liquid boiling. This effect can improve the capability of the liquid to absorb heat energy. These small bubbles are condensed back into liquid when the fluid is cooled. Convection is the most efficient method for heat removal and is used in a number of electronic applications where removal of large amounts of heat energy or high heat density are issues. The convective cooling can be implemented by open loop or close-loop approach. In the open loop approach, the moving fluid flows through the hot device and is discarded and replaced with fresh cooler fluid. In the close-loop approach, the coolant flow through the hot surface is passed through a “heat exchanger” which cools the fluid and then re-circulated back to the hot surface to be heated again. The most common example of open loop cooling is the forced or natural convection air-cooling. However, due to low specific heat properties of gases when large amounts of heat or high heat densities are an issue, liquid cooling is much more capable of meeting the cooling requirements.
Conventional re-circulating fluid cooling systems are composed of four major parts. The first is the heat sink used to remove the heat energy from the electronic component or assembly. The second part is a heat exchanger used to cool the fluid and transport this heat into the environment. The third part is a pump used to force the fluid to circulate through the cooling system. The fourth are the channels or plumbing used to transport the fluid though the cooling system.
All of these parts of the cooling system must be constructed to contain the fluid in the system for the life of the product. Conventional systems are composed of individual components that are assembled using various joining techniques where the interfaces between the components must be fluid tight. The fabrication and assembly of these individual components can be costly and relatively complicated. In addition, the interfaces between the various components are subject to leaking which will cause failure of the cooling system. These cost, assembly and reliability issues are addressed by the following invention.